1. Field of the Invention
The present invention relates to a two-component developing agent for use in a development of an electrostatic latent image formed on a photoconductor based on an electrophotography. In addition, the present invention is to provide an image forming apparatus in which the above two-component developing agent is used such that an image formation can be achieved at a high speed and good printing characteristics can be kept for a long time.
2. Description of the Related Art
Electrophotography known in the art includes a system described in U.S. Pat. No. 2,297,691 and the like. In this system, a photoconductor (such as photoconductive drum or the like) is generally used, an even electrostatic charge is provided on a surface of the photoconductor by means of corona discharge or the like. An optical image is applied onto the photoconductor by various means to form an electrostatic latent image thereon that is then developed with a fine powder called a toner.
If necessary, the toner image is transferred onto a recording medium, such as a paper. The toner image is then melted by means of pressing, heating, solvent vaporizing, light irradiating or the like so as to fix the toner image onto the recording medium, thereby providing a printing product. For example, an image forming apparatus for use in the above process includes a printer, a copying machine, a facsimile or the like.
As the developing agent for use in the image forming apparatus, there is known one-component developing agent comprising only a toner particle and a two-component developing agent comprising the toner particle and a carrier. In more recent years, a need exists for the image forming apparatus in which the image formation can be achieved at a higher speed. To this end, it is more suitable to use the two-component developing agent.
The above two-component developing agent comprises a carrier particle having a general magnetic character and an insulating toner particle. In a case where the development of electrostatic latent image is carried out on the photoconductor, only the toner is consumed while stirring the carrier and the toner in a developing container, so that the carrier can be reused. Since a predetermined voltage from the carrier is provided onto the toner with stirring, the toner is transferred to the photoconductor so as to form a toner image thereon. However, a balance between the charging and the discharging of both the toner and the carrier may be broken due to a stirring stress, thereby giving rise to the excessive charging, the stripping of a coating resin from a surface of the carrier and a change in an electric resistance due to filming of the toner onto the surface of the carrier.
Since the developability of the toner is dependent on a charge amount of the toner and a strength of an electric field at a developing region, an excessive increasing of a charging amount of the toner results in a decrease in the number of the toner attractive to the electrostatic latent image, thereby leading to a decrease in a printing density. In addition, with the stripping of the coating resin from the carrier surface and the filming with the toner, the electric resistance of the carrier may be increased. In such a case, the electric field for the development is weakened to deteriorate developability, thereby providing a lower printing density of a print. It is necessary to replace the two-component developing agent with a new one when a state of the toner and carrier becomes worse and the printing density is below an acceptable value.
On the other hand, in a high speed printer where the recording medium is transported at a speed more than 1 m/s and the printing is carried out more than 150 sheets per one minute, there exists a long-felt need for a long-range term of a replacement cycle of the two-component developing agent from the view points of maintenance and the low running cost.
To this end, Japanese Laid-Open Patent Application No. 7-72668 describes a two-component developing agent having a long lifetime by modification of a silicone resin with a fluorine atom. However, the technology disclosed in the above application results in a charge-up due to the excessive charging when printing at the high speed more than 1 m/s, thereby providing the inability of extending the lifetime of the carrier.
Additionally, an extensive study of a coverage of the carrier with the coating resin showed that good printing performance can be obtained with the relative low coverage. For example, Japanese Laid-Open Patent Application No. 4-188162 teaches an approach in which the coverage of the carrier with the coating resin is less than 12%. However, when printing at the higher speed with the developing agent disclosed in the Japanese Laid-Open Patent Application No. 4-188162, an increase in the electric resistance of the carrier can not be inhibited.
Japanese Laid-Open Patent Application Nos. 57-96355 and 1-29856 describe a more than 2 layers coating method by use of the silicone resin and a coating method by mixing different resins, respectively. However, when the image formation is carried out at the high speed by use of the above technologies, the stripping may occur at a boundary face between the resins on each surface of the carriers so that the above technologies can not be applied to the high speed printing based on the electrophotography.
Further, Japanese Laid-Open Patent Application No. 10-20562 teaches the use of the silicone resin having a uncured component of less than 4.3%. In addition, Japanese Patent No. 62-61948 describes the use of silicone resin having the uncured component of less than 30%. Under these conditions, the toner filming and the charge-up problems can not be inhibited sufficiently.
Therefore, the present invention has been made in view of the above-described problems of the prior art.
Accordingly, it is a general object of the present invention is to provide a two-component developing agent for use in an electrophotographic method and an image forming apparatus by use of the same wherein the foregoing problems are eliminated.
A more specific object of the present invention is to provide the two-component developing agent with which a high developability can be obtained when printing at a high speed and deterioration of developability can be prevented for even a long-term image formation period, and the image forming apparatus utilizing the above two-component developing agent.
The above objects of the present invention are achieved by a two-component developing agent for use in electrophotography, comprising:
a carrier comprising a magnetic particle, a surface of the magnetic particle coated with at least a resin; and
an insulating toner, wherein a mean particle size of the magnetic particle is between 30 and 90 xcexcm and an aggregation degree of said carrier is between 2 and 15%.
The above objects of the present invention are also achieved by a two-component developing agent for use in electrophotography, comprising:
a carrier comprising a magnetic particle, a surface of the magnetic particle coated with a resin comprising at least a conductive carbon and a cure-type (i.e., curable) silicone resin, preferably a modified silicone resin, more preferably a cure-type fluorine-modified silicone resin; and
an insulating toner,
wherein 70 to 95% of the surface of the magnetic particle is covered with a coating layer having a thickness of from 0.3 to 5 xcexcm and a soluble ratio in chloroform of the coating layer is between 5 and 20%, and further wherein a mean particle size of the magnetic particle is between 30 and 90 xcexcm and an aggregation degree of said carrier is between 2 to 15%.
According to one aspect of the present invention, there is provided an image forming apparatus in which an image formation is carried out with a two-component developing agent comprising: a carrier comprising a magnetic particle, a surface of said magnetic particle coated with a resin comprising at least a conductive carbon and a cure-type (i.e., curable) silicone resin, preferably a modified silicone resin, more preferably a cure-type fluorine-modified silicone resin; and an insulating toner, wherein 70 to 95% of the surface of the magnetic particle is covered with a coating layer having a thickness of from 0.3 to 5 xcexcm and a soluble ratio in chloroform of the coating layer is between 5 and 20%, and further wherein a mean particle size of the magnetic particle is between 30 and 90 xcexcm and an aggregation degree of said carrier is between 2 and 15%.
An advantage of the present invention is that it provides a two-component developing agent with which the charge-up due to an excessive charging can be prevented so as to give a stable toner charging during a long period of time.
The two-component developing agent in accordance with the present invention provides another advantage in that a long lifetime of a two-component developing agent can be obtained and the charge-up can be inhibited during a continuous printing.
Yet another advantage of the present invention is that a suitable charging of a two-component developing agent can be attained and abrasion resistance thereof can be improved.
A feature of the present invention is the provision of an image forming apparatus in which a good image formation can be obtained for a long-term.